Telecom networks include core networks, Metropolitan Area Networks (MANs) and access networks. A core network consists of core routers or switches and a backbone transmission network such as Synchronous Digital Hierarchy (SDH) network or Dense Wavelength Division Multiplexing (DWDM) network; an access network consists of various technologies such as Asymmetric Digital Subscriber Loop (ADSL) technology, Very High Bit Rate Digital Subscriber Loop (VDSL) technology, active point-to-point technology, PON technology and even wireless access technology.
With the emergence of new services, especially the development of high bandwidth video services such as Internet Protocol Television (IPTV) and High-Definition Television (HDTV), the access technology is challenged. The ADSL technology is economical but bandwidth-limited; the VDSL technology provides a bandwidth of over 50M but transmits a short distance. Besides, the ADSL technology and the VDSL technology are based on copper cables, thus vulnerable to interference. Moreover, with the copper resource becoming stringent, the development of the ADSL technology and the VDSL technology will be bottlenecked.
As a broadband optical access technology, the PON is characterized by a point-to-multipoint physical topology and consists of an Optical Line Terminal (OLT), a passive Optical Distribution Network (ODN) and multiple Optical Network Units (ONUs). Multiple ONUs share fiber resources and OLT ports; the ODN is connected to an OLT and one or more ONUs passively; the optical distribution point (ODP) in an ODN does not need any active node devices, but needs only a passive optical branching device (OBD). Therefore, the PON has these merits: shared bandwidth resources, economical investment on equipment rooms, high security of equipment, fast network construction and low overall cost of network construction.
A standardized PON includes: an Asynchronous Transfer Mode (ATM)-based PON (ATM-PON or APON), and a Broadband Passive Optical Network (BPON), both compliant with the International Telecommunications Union-Telecommunications (ITU-T) G983.x recommendations; an Ethernet PON (Ethernet-PON or EPON), compliant with the Institute of Electrical and Electronics Engineers (IEEE) 802.3ah recommendations; and a Gigabit-capable Passive Optical Network (GPON), compliant with the ITU-T G984.x recommendations. The PON technologies currently under research in the industry include: Wavelength Division Multiplexing (WDM)-based PON (WDM-PON), Optical Code Division Multiple Access (OCDMA)-based PON (OCDMA-PON), SubCarrier Multiplexed (SCM)-based PON (SCM-PON), etc.
With the growth of broadband services, the PON technologies are evolving. The process of evolution from APON/BPON to EPON and GPON is a process of increasing transmission bandwidth. Currently, the rate of the GPON is 2.5 Gbps for downstream and 2.5 Gbps, 1.5 Gbps or 622 Mbps for upstream. The frame structure of the GPON uses 125 μs (microsecond) as a period and uses the GFP as a link layer protocol, thus being suitable for transmitting not only Ethernet services data but also TDM services data. It is one of the ideal solutions for integrated services access.
In the current practice, the transmission mechanism for APON/BPON, GPON and EPON is Time Division Multiple Access (TDMA), namely, the downstream transmission is in the TDM mode and the upstream transmission is in the TDMA mode. In the TDMA mode, distance measurement (ranging) should be performed to control the time of sending upstream data from each ONU. However, the higher the PON rate is, the less accurate the ranging will be.
With the development of broadband services in the future, the optical access technologies will evolve to either of these possible destinations: The existing GPON or EPON evolve continuously to provide higher rates or combine with the WDM; or a wholly new technology such as WDM-PON replaces the existing technology. In view of the trend of the optical components, the system based on the WDM-PON technology is costly, and its room of development is rather limited before boom of the user quantity and brand services. Therefore, the TDM-PON evolving to high rates is much cost-effective and practical, and can inherit the existing GPON or EPON with respect to management and control, thus inheriting the technologies appropriately.
The GPON is a PON system promoted by the Full Service Access Network (FSAN) organization and formulated by the ITU-T standardization organization. The EPON challenging the BPON gives rise to the GPON. With respect to function and performance, the GPON is characterized by: providing multiple symmetric or asymmetric upstream and downstream rates flexibly, for example, upstream rate 1.244 Gbps, downstream rate 2.488 Gbps; the system distribution ratio is up to 1:16, 1:32, 1:64 or even 1:128, and is related to the Forward Error Correction (FEC) supported by GPON, while the EPON provides only symmetric 1.25 Gbps upstream and downstream rates, and provides a distribution ratio of up to 1:32; the GFP is suitable for adaptation of any data service; well supporting TDM service data transfer and providing assurance for timing performance; providing perfect Operation, Administration, Maintenance and Provisioning (OAM&P) capabilities.
The downstream frame of the GPON is a 125 μs frame structure. As shown in FIG. 1, a downstream frame in the GPON includes a Physical Control Block downstream (PCBd) overhead area and a payload area. The PCBd overhead area includes these fields: Physical Synchronization (PSync) field, superframe indication field, Physical Layer OAM downstream (PLOAMd) field, Bit Interleaved Parity (BIP) field, Payload Length downstream (PLend) field, and Upstream Bandwidth Map (US BW Map) field. The PSync field is used to implement synchronization between the ONU and the OLT; the PLOAMd field is used to bear the downstream Physical Layer OAM (PLOAM) information; the BIP field is used for error detection; the PLend field is used to indicate the length of the US BW Map field and the quantity of cells in the payload, wherein the PLend can occur twice in order to enhance the error tolerance; the US BW Map field is used to allocate upstream bandwidth, and includes an upstream timeslot indication overhead for indicating the start position and the end position of each ONU upstream timeslot. The control object of bandwidth allocation is Transmission Container (T-CONT). The OLT can allocate one or more T-CONTs to an ONU, which is a concept introduced into the PON Dynamic Bandwidth Allocation (DBA) technology to improve the efficiency of DBA.
The US BW Map field is used to indicate the start position and the end position of each ONU upstream timeslot; the T-CONT is the size of the timeslot allocated by the OLT to the ONU. The ONU sends an upstream burst packet to the OLT according to the position of the allocated timeslot. Taking ONU1 as an example, as shown in FIG. 2, through the US BW Map field in the downstream frame, the OLT notifies the ONU1 that the start position of the upstream timeslot is the 100th timeslot (TS100), and the end position is the 300th timeslot (TS300). In this way, the size of the T-CONT timeslots allocated by the OLT to the ONU1 is 200 timeslots. According to the timeslot position indicated by the received downstream frame, the ONU1 starts to send upstream burst packets to the OLT from TS100 of the upstream frame, and finishes sending the upstream burst packet at TS300.
In the GPON, the period of both upstream frames and downstream frames is 125 μs. As shown in FIG. 3A and FIG. 3B, each ONU sends an upstream burst packet from the T-CONT allocated by the OLT to the OLT. Such an upstream burst packet includes an overhead area and a payload area. The overhead area includes these fields: Physical Layer Overhead (PLOu), Physical Layer OAM upstream (PLOAMu) field, Physical Layer Sequence upstream (PLSu) field for adjusting power, and Dynamic Bandwidth Report upstream (DBRu) field. The PLOu field is used to implement burst synchronization, and includes a preamble, a delimiter and a BIP. After occupying the upstream channel, the ONU sends a PLOu unit to the OLT so that the OLT can get synchronized with the ONU quickly, and receive valid upstream data from the ONU correctly. The PLOAMu field is used to bear the upstream PLOAM information, and includes an ONU identifier (ONU ID), a message identifier (message ID), a message and a Cyclic Redundancy Code (CRC).
As described above, in a TDM-based GPON, the period of both upstream frames and downstream frames is 125 μs regardless of the upstream rate. In this way, all ONUs must finish receiving and handling downstream frames within 125 μs, and must finish sending the upstream burst packets on the corresponding timeslot position of the T-CONT allocated by the OLT to the ONU within 125 μs.
TABLE 1Time overhead of the ONU in the burst modeUpstreamEnableDisableTotalProtectionPreambleDelimitratetimetimetimetimetimetime(Mbps)(bit)(bit)(bit)(bit)(bit)(bit)155.52223261016622.0888641628201244.161616963244202488.3232321926410820RemarksMaximumMaximumMandatoryMinimumSuggestedSuggested
Table 1 shows that the OLT and all ONUs must finish handling all signals within 125 μs, and the time for all ONUs to send upstream burst packets must meet the time overhead specified in Table 1. As shown in Table 1, the enabling time and the disabling time of the ONU are very short. Taking the upstream rate 2488.32 Mbps as an example, the enabling time and the disabling time of the ONU is 32/2488.32M=13 ns (nanosecond), which imposes a strict requirement on the upstream time overhead such as enabling time and the disabling time of the ONU transmitting unit, and the preamble time of the burst. This brings difficulty to processing of circuits and manufacturing of transmitting units, and increases the cost of transmitting units obviously. Moreover, the requirement on performance of the OLT receiver is very strict, and it is required that the OLT must implement correct clock recovery and frame delimitation within a very short time, which brings difficulty to manufacturing of the OLT and increases the manufacturing cost.
In addition, the upstream timeslot indication overhead in the GPON is only 16 bits. If the start position and the end position of the upstream timeslot in the upstream mapping bandwidth are only 16 bits, the maximum timeslot quantity indicated by the GPON will be 216=65536, which is far from enough to meet the indication requirement of the upstream bandwidth position when the maximum timeslot quantity in the frame structure is greater than 65536 in the future.
In the final analysis, the data transmission mode in the TDM-PON in the prior art bottlenecks the development of the TDM-PON, and it is urgent to seek a data transmission solution that can boost the TDM-PON development.